Projection lens

ABSTRACT

A projection lens for projecting an image appearing on a cathode-ray tube onto a screen, small in residual aberrations, having a high imaging performance, also having a large aperture ratio and a wide field angle, suitable for a video projector having a high definition of, say, 1,125 scanning lines. The projection lens has, successively from the screen side, a Gauss type lens system, a positive lens mainly for enlarging the aperture ratio, and a negative lens mainly for widening the field angle, which negative lens has a strongly concave surface at the screen end.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a projection lens and more particularlyto a projection lens for a high definition video projector whichproduces on a screen an enlargement of an image appearing on a highdefinition cathode-ray tube.

2. Description of the Prior Art

Conventional projection lenses are useful for a cathode-ray tube (CRT)on which an image is formed by 525 scanning lines, and most of them donot have a color correction function. There are known some projectionlenses having an improved imaging capability attained by colorcorrection. One projection lens constituted by plastic lenses alone isknown as disclosed in Japanese Laid-Open Patent Application No.59-170812 which is constituted by four lens units each composed of fourlenses with aspherical surfaces. Further, a projection lens of a hybridconstruction (constituted by plastic and glass lenses), is known whichis constituted by four lens units each composed of five lenses, asdisclosed in Japanese Laid-Open Patent Application No. 58-139111.

However, with these conventional projection lenses having a colorcorrection function and constituted by plastic lenses, a good imagequality cannot be ensured because of a large focus drift caused by achange in the ambient temperature. On the other hand, with the hybridconstruction lenses which are less influenced by temperature variations,the aperture ratio is small, being in the range of 1:1.3 to 1:1.4, andso it is impossible to obtain a bright projected image. Further, in bothconstructions, because of large residual aberrations there has been aserious problem of deficiency of performance as a projection lens foruse in a video projector of a high definition having 1,125 scanninglines. Besides, because of a small half field angle of about 20° to 23°,the projection distance, namely, the distance from a screen to the foreend of the projection lens, must be large in order to obtain a largepicture plane. This requirement has been a problem in reducing the sizeof a projection television.

SUMMARY OF THE INVENTION

It is the object of the present invention to provide a projection lensfor a high definition video projector capable of accurately correctingand reducing residual aberrations and which is a wide field angleprojection lens having a large aperture ratio having a high performanceand at the same time satisfying the characteristics of light weight,compact construction and low cost which are contrary to thecharacteristics for aberration correction.

In order to achieve the above object, the projection lens of the presentinvention comprises, in order from the screen end, a Gauss type lenssystem, a positive lens unit, and a negative lens unit. Morespecifically, the projection lens of the present invention comprises,successively from the screen end, a first lens unit which is a positivelens, a second lens unit which is a positive meniscus lens having aconvex surface facing the screen end, a third lens unit which is anegative meniscus lens having a convex surface facing the screen end, afourth lens unit which is a negative meniscus lens having a stronglyconcave surface facing the screen end and having a cemented surface, afifth lens unit which is a positive lens unit, a sixth lens which is apositive lens mainly for attaining a large aperture ratio, and a seventhlens unit which is a negative lens mainly for attaining a wide fieldangle, having a strongly concave surface facing the screen end.

With this construction, residual abberations are corrected to asatisfactory extent and it becomes possible to obtain a performance as aprojection lens satisfactory for a video projector of a high definitionof 1,125 scanning lines. Moreover, there are attained a larger apertureratio in the range from 1:1.1 to 1:1.15 and a wide field angle exceeding26° in terms of a half field angle. Additionally, the positive lensesincluded in the first to fifth lens units can be made of glass having arefractive index not higher than 1.7 which is suitable for massproduction, and thus, when coupled with plastic lenses of the sixth andseventh group lens units, it is possible to attain a reduction of weightand cost. In other words, according to the present invention it ispossible to obtain a projection lens of light weight and low cost,having a performance satisfactory as a projection lens for a highdefinition video projector.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a projection lens according to a firstembodiment of the present invention;

FIGS. 2(a), 2(b) and 2(c) are characteristic diagrams of sphericalaberration, astigmatism and distortion, respectively, of the firstembodiment of the projection lens;

FIG. 3 is a schematic view of a projection lens according to a secondembodiment of the present invention; and

FIGS. 4(a), 4(b) and 4(c) are characteristic diagrams of sphericalaberration, astigmatism and distortion, respectively, of the secondembodiment of the projection lens.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the projection lens of the present invention, a positive lens unitmainly for attaining a large aperture ratio and a negative lens unitmainly for attaining a wide field angle are added at the CRT end of aGauss type lens system which is often used as a standard lens of asingle lens reflex camera, to thereby enhance the aberration correctingability and achieve a high performance projection lens having a largeaperture ratio and a wide field angle.

More specifically, a Gauss type lens system comprises first to fifthlens units I-V successively from the screen end. The first lens unit isa positive lens; the second lens unit is a positive meniscus lens havinga convex surface facing the screen end; the third lens unit is anegative meniscus lens having a convex surface facing the screen end;the fourth lens unit is a negative meniscus lens having a stronglyconcave surface facing the screen end and having a cemented surface; andthe fifth lens unit is a positive lens. Added thereto are a sixth lensunit VI mainly for attaining a large aperture ratio, the sixth lens unitbeing a positive lens, and a seventh lens unit VII mainly for attaininga wide field angle, the seventh lens unit being a negative lens. In thisconnection, the lens characteristics can be further improved by settingthe relation of the first to fifth lens units to the additional sixthand seventh lens units so as to satisfy the following conditions:

    1.2f<f.sub.1-5 <1.6f                                       (1)

    1.4f<f.sub.6 <1.7f                                         (2)

    0.1f<d.sub.11 <0.3f                                        (3)

    -1.4f<f.sub.7 <-f                                          (4)

Where,

f: focal length of the overall lens system

f₁₋₅ : focal length of the system (Gauss type lens system) from thefirst to the fifth lens units group

f₆ : focal length of the sixth lens unit

f₇ : focal length of the seventh lens unit

d₁₁ : interplane spacing between the fifth and sixth group lens units

Further, since the above additional (sixth and seventh) lens units arepositioned near the surface of a cathode ray tube, a large effectiveaperture is inevitably required, so by using plastic lenses there areattained a reduction of weight and of cost. Additionally, by making thesurfaces of the plastic lenses aspherical there are attained both asolution to the problem caused by a low index of the plastic lenses anda further improvement of performance.

The above condition (1) establishes the relation of the first to fifthlens units to the overall system. Outside the lower limit value thesystem will become more compact, but the spherical aberration flare willincrease, leading to deterioration of performance. And outside the upperlimit value the system will become larger in size and the burden ofaberration correction of the sixth group positive lens unit willincrease, thus making it difficult to achieve a high performance.

The condition (2) defines the focal length of the sixth group lens unit.Outside this condition the power distribution balance with the first tofifth lens units will be lost, so in that a good aberration correctioncan no longer be expected. More particularly, outside the lower limitthe spacing between the first to fifth lens units and the sixth lensunit will become larger to the extent that the correction of off-axisaberration is difficult, and outside the upper limit the power of thefirst to fifth lens units will become stronger and so it becomesdifficult to remove flare. The lower limit of the condition (2) alsoserves to maintain a focus point shift caused by a temperature variationwhich occurs when a plastic lens is used as the sixth lens unit, withina range free of any obstacle in practical use.

The condition (3) establishes the positional relation between the firstto fifth group lens units and the sixth lens unit. Outside the lowerlimit the correction of the aperture dependent aberration will becomeunsatisfactory, and a value exceeding the upper limit will result in areduced brightness of the edge of the image field and deterioration ofoff-axis aberration, although it is good for the correction of theaperture dependent aberration.

A good correction of the aperture dependent aberration has the advantageof increasing the Petzval sum, and in the construction of the first tosixth lens units it is difficult to make the Petzval sum small, so it isdesirable to add a field flattener.

The condition (4) concerns the focal length of the seventh lens unitwhich performs the function of the field flattener and contributes awidening of the field angle.

With the projection lens of the present invention, in order to utilizeits high imaging performance effectively, it is necessary to minimizestray light created by a surface reflection at the constituent lenses orthe face plate of the cathode-ray tube, and thereby prevent thedeterioration of contrast. The surface reflection of the lenses isimproved by the provision of an anti-reflection coating and that of theface plate of the cathode-ray tube is improved by filling the spacebetween the seventh lens unit and the CRT face plate F with a medium(e.g. an aqueous ethylene glycol solution or silicon gel) having arefractive index close to the refractive indices of the seventh lensunit and the CRT face plate. Therefore, it is desirable that the surfaceof the CRT face plate which is toward seventh lens unit be close toflat. The condition (4) is also a condition for the radius of curvatureof the concave surface of the seventh lens unit. Outside the lower limitit will be impossible to obtain the effect of decreasing the Petzval sumto a satisfactory extent, and a value exceeding the upper limit is notdesirable for off-axis aberration because the radius of curvature of theconcave surface will become smaller.

Concrete embodiments of the present invention will be describedhereinunder.

FIGS. 1 and 3 illustrate arrangements of projection lenses according tofirst and second embodiments, respectively, of the present invention,and FIGS. 2 and 4 are aberration diagrams of the first and secondembodiments, respectively. In the drawings, the reference characters r₁,r₂, r₃, . . . represent radii of curvature of the lens surfacessuccessively from the screen end; the characters d₁, d₂, d₃, . . .represent thicknesses and spacings of the constituent lenses; n₁, n₂,n₃, . . . represent refractive indices at line d of the lenses; ν₁, ν₂,ν₃, . . . represent Abbe's numbers at line d; f represents the focallength of the overall system; f₁₋₅ represents the focal length of thesystem from the first lens unit to the fifth group lens unit; f₆represents the focal length of the sixth lens unit; and f₇ representsthe focal length of the seventh lens unit. The * marked surfacesrepresent aspherical surfaces which are in the shape of arevolution-symmetric aspherical surface represented by the followingequation in a rectangular coordinate system which the X axis extends inthe direction of an optical axis, provided r is a vertex radius ofcurvature, k is a conical constant, and AD, AE, AF and AG are asphericalcoefficients: ##EQU1## Further, n₉ represents the refractive index of amedium filling the space between the seventh lens unit and the CRT faceplate, and F represents the CRT face plate.

FIRST EMBODIMENT

    __________________________________________________________________________               f = 148.264 mm    aperture ratio 1:1.1                             magnification of projection × 10.5                                                 f.sub.1-5 = 1.3173f                                                                             f.sub.6 = 1.4635f                                           f.sub.7 = -1.0979f                                                                              d.sub.11 = 0.2753f                               __________________________________________________________________________    first lens unit                                                               r.sub.1 = 170.31 mm                                                                          d.sub.1 = 14.22 mm                                                                         n.sub.1 = 1.67790                                                                         ν.sub.1 = 55.5                     r.sub.2 = 981.29 mm                                                                          d.sub.2 = 0.59 mm                                              second lens unit                                                              r.sub.3 = 93.32 mm                                                                           d.sub.3 = 18.53 mm                                                                         n.sub.2 = 1.69350                                                                         ν.sub.2 = 53.3                     r.sub.4 = 259.13 mm                                                                          d.sub.4 = 0.59 mm                                              third lens unit                                                               r.sub.5 = 275.33 mm                                                                          d.sub.5 = 3.54 mm                                                                          n.sub.3 = 1.54072                                                                         ν.sub.3 = 47.3                     r.sub.6 = 61.51 mm                                                                           d.sub.6 = 56.75 mm                                             fourth lens unit                                                              r.sub.7 = - 76.53 mm                                                                         d.sub.7 = 3.95 mm                                                                          n.sub.4 = 1.75520                                                                         ν.sub.4 = 27.7                     r.sub.8 = 4872.80 mm                                                                         d.sub.8 = 20.46 mm                                                                         n.sub.5 = 1.69350                                                                         ν.sub.5 = 53.3                     r.sub.9 = -98.26 mm                                                                          d.sub.9 = 0.28 mm                                              fifth lens unit                                                               r.sub.10 = 291.93 mm                                                                         d.sub.10 = 18.39 mm                                                                        n.sub.6 = 1.69350                                                                         ν.sub.6 = 53.3                     r.sub.11 = -215.90 mm                                                                        d.sub.11 = 40.82 mm                                            sixth lens unit                                                               r.sub.12 = 264.48 mm                                                                         d.sub.12 = 30.60 mm                                                                        n.sub.7 = 1.49178                                                                         ν.sub.7 = 57.7*                    r.sub.13 = -172.05 mm                                                                        d.sub.13 = 44.10 mm      *                                     seventh lens unit                                                             r.sub.14 = -80.05 mm                                                                         d.sub.14 = 3.95 mm                                                                         n.sub.8 = 1.49178                                                                         ν.sub.8 = 57.7*                    r.sub.15 = ∞                                                                           d.sub.15  = 16.92 mm                                                                       n.sub.9 = 1.45                                    r.sub.16 = ∞                                                                           d.sub.16 = 11.62 mm                                                                        n.sub.10 = 1.50                                   r.sub.17 = -2350.00 mm                                                        __________________________________________________________________________    Aspherical coefficient                                                                  K    AD       AE      AF       AG                                   __________________________________________________________________________    twelfth surface:                                                                        -1.4213                                                                            -0.1833 × 10.sup.-7                                                              0.8328 × 10.sup.-11                                                             -0.6460 × 10.sup.-15                                                              0.1536 × 10.sup.-18           thirteenth surface:                                                                      1.3146                                                                            -0.3321 × 10.sup.-7                                                              0.2188 × 10.sup.-11                                                              0.1849 × 10.sup.-14                                                             -0.8994 × 10.sup.-19           fourteenth surface:                                                                      0.6996                                                                            -0.1432 × 10.sup.-6                                                              0.3929 × 10.sup.-10                                                              0.6206 × 10.sup.-15                                                             -0.2109 × 10.sup.-18           __________________________________________________________________________

SECOND EMBODIMENT

    __________________________________________________________________________              f = 149.128 mm     aperture ratio 1:1.15                            magnification of projection × 10.5                                                f.sub.1-5 = 1.4815f                                                                              f.sub.6 = 1.5188f                                          f.sub.7 = -1.2940f d.sub.11 = 0.1300f                               __________________________________________________________________________    first lens unit                                                               r.sub.1 = 293.62 mm                                                                          d.sub.1 = 11.20 mm                                                                        n.sub.1 = 1.67790                                                                         ν.sub.1 = 55.5                      r.sub.2 = ∞                                                                            d.sub.2 = 0.58 mm                                              second lens unit                                                              r.sub.3 = 94.42 mm                                                                           d.sub.3 = 15.58 mm                                                                        n.sub.2 = 1.69350                                                                         ν.sub.2 = 53.3                      r.sub.4 = 176.36 mm                                                                          d.sub.4 = 1.49 mm                                              third lens unit                                                               r.sub.5 = 195.87 mm                                                                          d.sub.5 = 11.73 mm                                                                        n.sub.3 = 1.54072                                                                         ν.sub.3 = 47.3                      r.sub.6 = 62.58 mm                                                                           d.sub.6 = 67.78 mm                                             fourth lens unit                                                              r.sub.7 = -70.29 mm                                                                          d.sub.7 = 3.89 mm                                                                         n.sub.4 = 1.75520                                                                         ν.sub.4 = 27.7                      r.sub.8 = 5129.85 mm                                                                         d.sub.8 = 20.69 mm                                                                        n.sub.5 = 1.69350                                                                         ν.sub.5 = 53.3                      r.sub.9 = -88.89 mm                                                                          d.sub.9 = 1.14 mm                                              fifth lens unit                                                               r.sub.10 = 271.21 mm                                                                         d.sub.10 = 27.21 mm                                                                       n.sub.6 = 1.69350                                                                         ν.sub.6 = 53.3                      r.sub.11 = -244.09 mm                                                                        d.sub.11 = 19.38 mm                                            sixth lens unit                                                               r.sub.12 = 292.07 mm                                                                         d.sub.12 = 33.23 mm                                                                       n.sub.7 = 1.49178                                                                         ν.sub.7 = 57.7*                     r.sub.13 = -173.30 mm                                                                        d.sub.13 = 63.57 mm     *                                      seventh lens unit                                                             r.sub.14 = -94.90 mm                                                                         d.sub.14 = 3.89 mm                                                                        n.sub.8 = 1.49178                                                                         ν.sub.8 = 57.7                      r.sub.15 = ∞                                                                           d.sub.15 =  17.10 mm                                                                      n.sub.9 = 1.45                                     r.sub.16 = ∞                                                                           d.sub.16 = 11.62 mm                                                                       n.sub.10 = 1.50                                    r.sub.17 = -2350.00 mm                                                        __________________________________________________________________________    Aspherical coefficient                                                                 K    AD       AE      AF       AG                                    __________________________________________________________________________    twelfth surface:                                                                       -0.2401                                                                            -0.1110 × 10.sup. -7                                                             0.1044 × 10.sup.-10                                                             -0.2219 × 10.sup.-15                                                              0.4493 × 10.sup.-19            thirteenth surface:                                                                    -0.6534                                                                             0.4773 × 10.sup.-8                                                              0.8093 × 10.sup.-11                                                              0.9191 × 10.sup.-15                                                             -0.8274 × 10.sup.-19            __________________________________________________________________________

What is claimed is:
 1. A projection lens for producing on a screen an enlargement of an image appearing on a cathode-ray tube (CRT), comprising: successively from the screen end, a Gauss type lens system, a positive lens unit contributing mainly to enlarging the aperture ratio and comprising at least one lens; and a negative lens unit contributing mainly to widening the field angle and having a strongly concave surface which faces the screen end, said projection lens satisfying the following conditions:

    1.2f<f.sub.1-5 <1.6f                                       (1)

    1.4f<f.sub.6 <1.7f                                         (2)

    0.1f<d.sub.11 <0.3f                                        (3)

    -1.4f<f.sub.7 <-f                                          (4)

where, f: focal length of the overall system f₁₋₅ : focal length of the Gauss type lens system f₆ : focal length of the positive lens unit contributing mainly to enlarging the aperture ratio f₇ : focal length of the negative lens unit contributing mainly to widening the field angle d₁₁ : interplane spacing between the Gauss type lens system and the positive lens unit contributing mainly to enlarging the aperture ratio.
 2. The projection lens according to claim 1, having substantially the following characteristics:

    __________________________________________________________________________                f = 148.264 mm    aperture ratio 1:1.1                              magnification of projection × 10.5                                                  f.sub.1-5 = 1.3173f                                                                              f.sub.6 = 1.4635f                                            f.sub.7 = -1.0979f                                                                               d.sub.11 = 0.2753f                                __________________________________________________________________________     first lens unit                                                                r.sub.1 = 170.31 mm                                                                           d.sub.1 = 14.22 mm                                                                          n.sub.1 = 1.67790                                                                          ν.sub.1 = 55.5                      r.sub.2 = 981.29 mm                                                                           d.sub.2 = 0.59 mm                                               second lens unit                                                               r.sub.3 = 93.32 mm                                                                            d.sub.3 = 18.53 mm                                                                          n.sub.2 = 1.69350                                                                          ν.sub.2 = 53.3                      r.sub.4 = 259.13 mm                                                                           d.sub.4 = 0.59 mm                                               third lens unit                                                                r.sub.5 = 275.33 mm                                                                           d.sub.5 = 3.54 mm                                                                           n.sub.3 = 1.54072                                                                          ν.sub.3 = 47.3                      r.sub.6 = 61.51 mm                                                                            d.sub.6 = 56.75 mm                                              fourth lens unit                                                               r.sub.7 = - 76.53 mm                                                                          d.sub.7 = 3.95 mm                                                                           n.sub.4 = 1.75520                                                                          ν.sub.4 = 27.7                      r.sub.8 = 4872.80 mm                                                                          d.sub.8 = 20.46 mm                                                                          n.sub.5 = 1.69350                                                                          ν.sub.5 = 53.3                      r.sub.9 = -98.26 mm                                                                           d.sub.9 = 0.28 mm                                               fifth lens unit                                                                r.sub.10 = 291.93 mm                                                                          d.sub.10 = 18.39 mm                                                                         n.sub.6 = 1.69350                                                                          ν.sub.6 = 53.3                      r.sub.11 = -215.90 mm                                                                         d.sub.11 = 40.82 mm                                             sixth lens unit                                                                r.sub.12 = 264.48 mm                                                                          d.sub.12 = 30.60 mm                                                                         n.sub.7 = 1.49178                                                                          ν.sub.7 = 57.7*                     r.sub.13 = -172.05 mm                                                                         d.sub.13 = 44.10 mm      *                                      seventh lens unit                                                              r.sub.14 = -80.05 mm                                                                          d.sub.14 = 3.95 mm                                                                          n.sub.8 = 1.49178                                                                          ν.sub.8 = 57.7*                     r.sub.15 = ∞                                                                            d.sub.15  = 16.92 mm                                                                        n.sub.9 = 1.45                                     r.sub.16 = ∞                                                                            d.sub.16 = 11.62 mm                                                                         n.sub.10 = 1.50                                    r.sub.17 = -2350.00 mm                                                         __________________________________________________________________________     Aspherical coefficient                                                                   K    AD       AE      AF       AG                                    __________________________________________________________________________     twelfth surface:                                                                         -1.4213                                                                             -0.1833 × 10.sup.-7                                                               0.8328 × 10.sup.-11                                                              -0.6460 × 10.sup.-15                                                               0.1536 × 10.sup.-18            thirteenth surface:                                                                       1.3146                                                                             -0.3321 × 10.sup.-7                                                               0.2188 × 10.sup.-11                                                               0.1849 × 10.sup.-14                                                              -0.8994 × 10.sup.-19            fourteenth surface:                                                                       0.6996                                                                             -0.1432 × 10.sup.-6                                                               0.3929 × 10.sup.-10                                                               0.6206 × 10.sup.-15                                                              -0.2109 × 10.sup.-18            __________________________________________________________________________

where r₁, r₂, r₃, . . . represent radii of curvature of the lens surfaces successively from the screen end; d₁, d₂, d₃, . . . represent thicknesses and spacings of the lenses; n₁, n₂, n₃, . . . represent refractive indices at line d of the lenses; ν₁, ν₂, ν₃, . . . represent Abbe's numbers at line d; f represents the focal length of the overall system; f₁₋₅ represents the focal length of the Gauss type lens system comprising the first to fifth lens units; f₆ represents the focal length of the sixth lens unit; and f₇ represents the focal length of the seventh lens unit; the * marked surfaces represent aspherical surfaces which are in the shape of a revolution-symmetric aspherical surface represented by the following equation in a rectangular coordinate system with the X axis extending in the direction of the optical axis, provided r is a vertex radius of curvature, k is a conical constant, and AD, AE, AF and AG are aspherical coefficients: ##EQU2## n₉ represents the refractive index of a medium filled in the space between the seventh lens unit and the face plate of the cathode-ray tube; and F represents the face plate of the cathode-ray tube.
 3. The projection lens according to claim 1, having substantially the following characteristics:

    __________________________________________________________________________               f = 149.128 mm     aperture ratio 1:1.15                             magnification of projection × 10.5                                                 f.sub.1-5 = 1.4815f                                                                               f.sub.6 = 1.5188f                                           f.sub.7 = -1.2940f d.sub.11 = 0.1300f                                __________________________________________________________________________     first lens unit                                                                r.sub.1 = 293.62 mm                                                                           d.sub.1 = 11.20 mm                                                                         n.sub.1 = 1.67790                                                                          ν.sub.1 = 55.5                       r.sub.2 = ∞                                                                             d.sub.2 = 0.58 mm                                               second lens unit                                                               r.sub.3 = 94.42 mm                                                                            d.sub.3 = 15.58 mm                                                                         n.sub.2 = 1.69350                                                                          ν.sub.2 = 53.3                       r.sub.4 = 176.36 mm                                                                           d.sub.4 = 1.49 mm                                               third lens unit                                                                r.sub.5 = 195.87 mm                                                                           d.sub.5 = 11.73 mm                                                                         n.sub.3 = 1.54072                                                                          ν.sub.3 = 47.3                       r.sub.6 = 62.58 mm                                                                            d.sub.6 = 67.78 mm                                              fourth lens unit                                                               r.sub.7 = -70.29 mm                                                                           d.sub.7 = 3.89 mm                                                                          n.sub.4 = 1.75520                                                                          ν.sub.4 = 27.7                       r.sub.8 = 5129.85 mm                                                                          d.sub.8 = 20.69 mm                                                                         n.sub.5 = 1.69350                                                                          ν.sub.5 = 53.3                       r.sub.9 = -88.89 mm                                                                           d.sub.9 = 1.14 mm                                               fifth lens unit                                                                r.sub.10 = 271.21 mm                                                                          d.sub.10 = 27.21 mm                                                                        n.sub.6 = 1.69350                                                                          ν.sub.6 = 53.3                       r.sub.11 = -244.09 mm                                                                         d.sub.11 = 19.38 mm                                             sixth lens unit                                                                r.sub.12 = 292.07 mm                                                                          d.sub.12 = 33.23 mm                                                                        n.sub.7 = 1.49178                                                                          ν.sub.7 = 57.7*                      r.sub.13 = -173.30 mm                                                                         d.sub.13 = 63.57 mm     *                                       seventh lens unit                                                              r.sub.14 = -94.90 mm                                                                          d.sub.14 = 3.89 mm                                                                         n.sub.8 = 1.49178                                                                          ν.sub.8 = 57.7                       r.sub.15 = ∞                                                                            d.sub.15 =  17.10 mm                                                                       n.sub.9 = 1.45                                      r.sub.16 = ∞                                                                            d.sub.16 = 11.62 mm                                                                        n.sub.10 = 1.50                                     r.sub.17 = -2350.00 mm                                                         __________________________________________________________________________     Aspherical coefficient                                                                  K    AD       AE      AF       AG                                     __________________________________________________________________________     twelfth surface:                                                                        -0.2401                                                                             -0.1110 × 10.sup. -7                                                              0.1044 × 10.sup.-10                                                              -0.2219 × 10.sup.-15                                                               0.4493 × 10.sup.-19             thirteenth surface:                                                                     -0.6534                                                                              0.4773 × 10.sup.-8                                                               0.8093 × 10.sup.-11                                                               0.9191 × 10.sup.-15                                                              -0.8274 × 10.sup.-19             __________________________________________________________________________

where r₁, r₂, r₃, . . . represent radii of curvature of the lens surfaces successively from the screen end; d₁, d₂, d₃, . . . represent thickness and spacings of the lenses; n₁, n₂, n₃, . . . represent refractive indices at line d of the lenses; ν₁, ν₂, ν₃, . . . represent Abbe's numbers at line d; f represents the focal length of the overall system; f₁₋₅ represents the focal length of the Gauss type lens system comprising the first to fifth lens units; f₆ represents the focal length of the sixth lens unit; f₇ represents the focal length of the seventh lens unit; the * marked surfaces represent aspherical surfaces which are in the shape of a revolution-symmetric aspherical surface represented by the following equation in a rectangular coordinate system with the X axis extending in the direction of the optical axis, provided r is a vertex radius of curvature, k is a conical constant, and AD, AE, AF and AG are aspherical coefficients: ##EQU3## n₉ represents the refractive index of a medium filled in the space between the seventh lens unit and the face plate of the cathode-ray tube; and F represents the face plate of the cathode-ray tube.
 4. A projection lens for producing on a screen an enlargement of an image appearing on a cathode-ray tube (CRT), comprising: successively from the screen end, a first lens unit which is a positive lens; a second lens unit which is a positive meniscus lens having a convex surface facing the screen end; a third lens unit which is a negative meniscus lens having a convex surface facing the screen end; a fourth lens unit which is a negative meniscus lens having a strongly concave surface facing the screen end and having a cemented surface; fifth and sixth lens units which are both positive lenses; and a seventh lens unit which is a negative lens having a strongly concave surface facing the screen end, said projection lens satisfying the following four conditions:

    1.2f<f.sub.1-5 <1.6f                                       (1)

    1.4f<f.sub.6 <1.7f                                         (2)

    0.1f<d.sub.11 <0.3f                                        (3)

    -1.4f<f.sub.7 <-f                                          (4)

where, f: focal length of the overall system f₁₋₅ : focal length of the system from the first to fifth lens units f₆ : focal length of the sixth lens unit f₇ : focal length of the seventh lens unit d₁₁ : interplane spacing between the fifth and sixth lens units. 